Perceived iris color in humans varies widely between individuals. Iris color of a normal human is dependent on the thickness of collagen fibers on the stroma and the presence and density of colored pigment in the melanin cells of the anterior border layer (or anterior stroma) of the iris. The pigment may exist in more than one form, such as the brown pigment eumelanin and the yellow pigment pheomelanin. These pigments are typically present in both the anterior border layer of the iris and in the deeper epithelial layers of the iris. These pigments may not, however, be evenly distributed between these two layers. For example, pheomelanin may be more prominent in the epithelium than in the anterior border layer.
The stroma consists principally of a network of gray collagen fibers and red capillaries. There is no brown, green or blue in the stroma. The stroma creates the appearance of blue color by refracting the light that passes through its collagen fibers, in much the same way that the refraction of the sun's light by atmospheric molecules creates the appearance of a blue sky (the so-called “Rayleigh scattering”). The thinner the collagen fibers, the more the redness of the capillaries is perceived, producing a violet appearance. The thicker the fibers, the more the grayness of the fibers is perceived, producing a gray appearance. A blue iris is presented in most individuals. Violet and gray irises are rare.
The anterior border layer is anterior to (or in front of) the stroma. The epithelium is posterior to (behind) the stroma. In rare cases, such as albinism, there is no pigment in either the epithelium or the anterior border layer, so the vascular components of the posterior chamber of the eye are revealed, and the iris presents a red or pink color. In most individuals, however, the epithelium is highly pigmented, so the vascular components of the posterior chamber are occluded, but the presence or density of pigment in the anterior border layer varies considerably, so the blue (or gray or violet) light produced by the stroma may or may be fully revealed, partially revealed, or fully occluded. If little or no pigmented is present in the anterior border layer, the blue (or gray or violet) light produced by the stroma is fully revealed, and the perceived iris color is blue (or gray or violet). If the anterior border layer is densely pigmented, the stroma is fully occluded, and the iris appears brown. (In some cases, pigment density is so great that the iris appears almost black in color.) If pigment density is low-intermediate, then the blue light produced by the stroma is only slightly occluded by the yellow-brown melanin cells, and this combination of blue and yellow presents a green iris. If cell density is high-intermediate, then the blue light produced by the stroma is more occluded by the yellow-brown melanin cells, and this combination of blue and yellow-brown presents a hazel (green-brown) iris.
Iris color plays and has played a significant social function as an attribute of beauty. In recent years, alteration of some types of iris color has become possible through the use of colored contact lenses. Such colored contact lenses can have a prescriptive optical power, or can be optically neutral such that the lenses serve as a cosmetic function only.
There are several disadvantages associated with the use of colored contact lenses for cosmetic purposes. First, the lenses have the same potential complications of use as contact lenses with prescriptive optical powers, including allergic reactions to the lens material and infections from improper handling. Further, contact lenses cannot be tolerated by some potential users due to the discomfort. Additionally, colored contact lenses require a degree of dexterity to insert and remove that is not possessed by all potential users. Further, permanent changes in iris color cannot be achieved through the use of colored contact lenses. Moreover, colored contact lenses often fail to provide a natural-looking (and therefore cosmetically acceptable) effect. Whereas natural brown irises are opaque, natural blue (or gray or violet) and green irises are not. When a blue (or gray or violet) and green contact lens is placed over a brown iris, however, an opaque blue (or gray or violet) and green iris is presented. Because this condition does not exist in nature, the iris appears fake and contrived and is not cosmetically appealing.
Another method for altering iris color involves the use of colored lenses implanted anteriorly to the iris. Such implants require an invasive procedure to place the lens in position. Because of the potential complication of an invasive procedure and of leaving a foreign body within the eye, the implantation of color lenses has not become a widely adopted procedure.
Therefore, there remains a need for a method to alter iris pigment in a human which does not require colored contact lenses or implanted lenses. Additionally, there remains a need for techniques to alter iris pigment in a human permanently.